DC-DC converters which output high voltage pulses in response to pulses of substantially lower potential are known in the art. Such a converter, used in the supply of a high voltage capacitor ignition device for internal combustion engines, is for example discussed in my earlier U.S. Pat. No. 4,696,280. The pulse generator of that DC-DC converter is controlled by a pulse-width modulator and includes two power transistors acting in push-pull operation on separate primary transformer windings. A full-wave bridge rectifier connected to the secondary transformer winding supplies a high DC voltage to an ignition capacitor which forms the charging capacitor.
When, in the use of such a high-voltage capacitor ignition device, the sparking is intensified and the maximum spark frequency at the same time increased then considerably more power is passed through the capacitor ignition device, as a result of which correspondingly greater heat must be dissipated. When a capacitor ignition device of relatively small dimensions is arranged in a correspondingly small housing, as is common, the additionally required heat dissipation becomes a problem.
In such a capacitor ignition device the DC-DC converter periodically charges the charging capacitor, at the frequency of the required train of sparks, each time from zero potential to a predetermined charge voltage. Initially the discharged charging capacitor constitutes a short circuit for the DC-DC converter after which, with increasing charging voltage, the load of the converter decreases since the load impedance represented by the charging capacitor is increasing and, therefore, the initial mismatch between the DC-DC converter and the load impedance is gradually reduced.
With increasing spark frequency, the charge voltage of the charging capacitor decreases. With a large transformer stepup ratio of about 25, the continuous mismatch between load impedance and the DC-DC converter also impairs and decreases the operating efficiency of the converter. In practice, carefully dimensioned capacitor ignition devices have an efficiency of only about 45% at low output, which decreases by approximately a third or more--namely to below 30%--at maximum spark frequency and work. A low efficiency at high output, however, correspondingly requires the dissipation of large amounts of heat which, in turn, limits the maximum power of the capacitor ignition device and unnecessarily burdens the power supply of the automobile or other vehicle electrical system.
It is accordingly the desideration of the invention to provide an improved DC-DC converter for generating, in response to a low voltage pulse train, a pulsed high voltage output with variable matching to the varying load impedance of the charging capacitor and thereby substantially increase the operating efficiency of the converter.